A simple retardation-switch of the prior art includes a single variable birefringence liquid-crystal (LC) device. This can be implemented using an electrically controlled birefringence (ECB) device, which is an anti-parallel aligned nematic LC. Or it could be a vertically aligned nematic (VAN) LC, or a pi-cell, which is a parallel-aligned nematic LC. It could even be a mixed-mode nematic such as a 64° twist nematic which for a particular design is known to produce a circular polarization.
Nematic LC devices switch retardation by changing the integrated projection of the optic-axis onto the plane of the substrate. For a typical ECB, the device behaves substantially as a positive uniaxial (+A-plate) retarder at zero volts, and has virtually zero in-plane retardation at a sufficiently high voltage. In many applications, the normal-incidence retardation is preferably preserved over a range of incident cone angles. Devices such as the ECB or VAN device are relatively attractive because they can be operated with very little excess retardation (i.e. retardation that is unmodulated, or passive). And with sufficiently high switching voltage, these devices can suffer very little angle dependence associated with the splay at the substrate boundary. Conversely, the pi-cell can have high passive retardation (e.g. ⅔ to ⅚ of the total cell retardation), and can show a high degree of splay in the low-voltage state. As such, the pi-cell can have relatively poor retardation uniformity off-normal in both high and low voltage states.
It is against this background that the present invention has been developed.